The long-term objective of this project is to understand how spinal cord mechanisms are adapted in different regions of the spinal cord to subserve different kinds of movement or functions. This will be studied by examining organization within the neck segments, for muscles involved in head movements. The results will be compared to the extensive knowledge of spinal mechanisms in the hindlimb segments, as well as to the scarcer observations on organization in other regions. The experiments will focus on the reflexes and presynaptic actions of group I muscle afferents (Ia from spindles, Ib from Golgi tendon organs), since the most detailed comparative observations available concern these actions. However, information on actions of other afferents (from muscle or skin), and on other mechanisms (recurrent effects), will be obtained in the course of the experiments, to extend knowledge of the segmental control of head movements. It is hypothesized that presynaptic actions (which are exerted on other afferents) assist reflex actions in producing specific movements. This would mean that specific patterns of presynaptic actions should match specific patterns of reflex actions, as would be revealed by describing the properties and numbers of neuronal subsystems mediating the actions. It would also mean that presynaptic and reflex actions should vary together in different regions of the spinal cord, to serve different movements. The experiments will be performed on anesthetized cats. Intracellular and intraaxonal recording will be used to study what actions occur: particularly, whether Ib reflexes, Ia reciprocal inhibition, and the Ia reflexes mimicking Ib reflexes occur in the neck segments as in the hindlimb. The distribution of presynaptic actions will be investigated; the study of Ia synergisms extended. Electrical stimulation of a variety of nerves, spatial facilitation methods, and selective activation of Ia and Ib afferents, will be used to characterize underlying neuronal systems, in terms of patterns of convergent inputs for specific targets. Distributions of effects, the neuronal systems described, will be examined for parallelisms or assymetries of actions of different spinal mechanisms. Variations in segmental control will be compared to variations in descending control. By examining regional adaptation of reflexes and spinal mechanism controlling them, this project will provide further insight into the way movements and posture are normally produced and controlled.